This invention relates to electron guns for color picture tubes, and more particularly to an in-line type electron gun for a color picture tube in which the focusing characteristic is improved.
In general, the diameter of the main lens of the electron gun for a color picture tube affects its focusing characteristic to a great extent, and thus, in order to obtain a fine focusing characteristics, it is desirable to maximize the diameter of the main lens.
FIG. 1 is a longitudinal sectional view of a main part of one example of a prior art in-line type electron gun of a bipotential focusing system. Referring to FIG. 1, there are shown cathodes 1A, 1B and 1C for emitting three electron beams from their head surfaces, a control electrode 2 for controlling the amounts of the electron beams, an accelerating electrode 3 for accelerating the electron beams, and a lower focusing electrode 4 for focusing the electron beams. The control electrode 2, accelerating electrode 3 and lower focusing electrode 4 have sets of electron beam passing holes 2A, 2B and 2C, 3A, 3B and 3C and 4A, 4B and 4C for the three electron beams, respectively. In addition, there are shown an upper focusing electrode 5 having electron beam passing holes 5A, 5B and 5C, and an anode 6 having electron beam passing holes 6A, 6B and 6C. The three electron beam passing holes of each of the upper focusing electrode and anode are formed by a process of drawing and arranged in line to oppose to each other. When each of the anode 6 and upper focusing electrode 5 is applied with a predetermined potential, three main electrostatic focusing lenses (hereafter simply referred to as main lenses) for the three electron beams are established at the opposed electron beam passing holes. For example, the upper focussing electrode 5 and the anode 6 are applied with about 7,000 volts and 25,000 volts, respectively. The respective electrodes having the electron beam passing holes are made of thin plates by the process of drawing so as to form an oval cup shape, as shown in FIG. 1.
In the electron gun constructed as above, the three electron beams A, B and C, which are controlled in their amounts by the signal potentials applied to the three cathodes 1A, 1B and 1C, are slightly focused by prefocus lenses formed between the opposed holes of the accelerating electrode 3 and lower focusing electrode 4. Then, the electron beams are focused by the main lenses formed by the upper focusing electrode 5 and anode 6 so that beam spots are projected on a phosphor screen of the picture tube not shown. At the same time, the outer sides of electron beams A and C are tilted by an angle .theta. to the center beam by a known measure such that the electron beam passing holes 6A and 6C of the anode 6 are made slightly eccentric to the outside with respect to the electron beam passing holes 5A and 5C of the upper focusing electrode 5, so that the three electron beams A, B and C are converged at a point. Reference numeral 7 denotes a convergence electrode.
In the electron gun constructed as above, the beam spot size on the phosphor screen of picture tube affects sharpness of the picture and is therefore desired to be as small as possible. In general, the diameter of the main lens is increased for improving the focusing characteristic to this end.
FIG. 2 is a plan view showing a main part of the upper focusing electrode 5 as viewed from the anode 6. The surface of the anode 6 opposing the upper focusing electrode 5 also has an identical configuration. Referring to FIG. 2, the three electron beam passing holes 5A, 5B and 5C have a diameter D and are arranged in line to be separated by a center-to-center spacing S. In order to increase the diameters of the main lenses for the improvement of the focusing characteristic, it is necessary to increase the diameter D of the electron beam passing holes 5A, 5B and 5C. Since the electron beam passing holes 5A, 5B and 5C of the upper focusing electrode 5 are made of a non-magnetic metal plate in an about 0.3 mm thick, for example, stainless steel plate and formed by pressing, these holes must be subjected to drawing in order to improve the breakdown voltage characteristic between the focusing electrode 5 and the anode 6 shown in FIG. 1. Thus, edges of these holes can be rounded by drawing to advantageously suppress a tendency of discharge. Further, to prevent the deterioration of rotational symmetry of the main lens electric field due to a non-symmetrical shape of the oval cup shape electrode, the depth, l, for drawing is required to be 1/2 or more of the diamater D of the hole. But, drawing the thin plate is so critical as to limit the diameter D to a dimension which is smaller than the center-to-center spacing S by 0.8 to 1.0 mm. the increase of the diameter D will therefore require increase of the spacing S. The increase of the spacing S will however result in large convergence error at each point of the phosphor screen upon operation of picture tube and in an increase of the dimension in the direction of the holes disposition, orthogonal to the tube axis, of the upper focusing electrode 5 and anode 6 which form the main lenses, so that they become close to the inner wall of the bulb neck within which the electron gun is placed, thus the breakdown voltage characteristic is deteriorated.
Further, in order to obtain a good focusing characteristic, it is the practice that the allowable deformity of a circle (major axis-minor axis) for the electron beam passing holes 5A, 5B and 5C is desired to be about 0.5% or less of the hole diameter D. Accordingly, in assembling the electron gun, the respective electrodes are supported by a tool (not shown) provided with three core stems passing through the respective electron beam passing holes, and heated multiform glass 8 (bead glass) are pressed against supporting members 9. In this case, in consideration of allowable production errors of the center-to-center spacing S and the hole diameter D in the respective electrodes, the three core steams are made thinner than the hole diameter D by about 0.02 to 0.03 mm. Consequently, stress generated by pressing the multiform glass 8 causes the respective cup-shaped electrodes with production errors to deform. The thus deformed upper focusing electrode affects the electron beam passing holes 5A, 5B and 5C formed by drawing, with the result that, in measurement of the electron gun after removing the same from the tool, out of roundness are about 0.05 mm in extremities, amounting to about 1.3% of the hole diameter D which is 3.9 mm, for example. With the drawn holes having worse roundness, the main lens electric fields are deformed to cause an astigmatic aberration in the electron beams, thereby giving rise to fatal deterioration of the focusing characteristic.
In order to extend the diameter of main lens, one of the inventors of the present invention, Yukihiro Izumida, has made a proposal in U.S. patent application Ser. No. 558,277 entitled "Electron Gun for Color Picture Tube" and filed on Dec. 5, 1983. According to this proposal, for the sake of maximizing the diameters of the main lenses within a limited range of the electrode dimension, the electron beam passing holes for forming the lenses are shaped into an oval hole having a major axis in a direction orthogonal to the direction in which the electron beam passing holes are arranged, and asymmetry of electric fields to the tube axis due to the oval holes is corrected by varying the distance between the opposed anode and upper focusing electrode, whereby a thin plate can be drawn to form the beam passing holes therein whose lengths in the major axes are equivalent to increased effective diameters of the lenses. However, such an electrode processed by drawing does not provide a solution of the probelm that the beam passing holes are deformed in production of the electron gun.